The goal of this investigation is to clarify by means of genetic analysis two important aspects of the biology of RNA tumor viruses. 1) the mechanism of genetic recombination, 2) aspects of the formation of the provirus. Since recombination is more easily recognized if it does affect the gene responsible for transformation (src) our early and present efforts are centered on the genetic structures of the src gene. We are in the process of mapping a number of temperature sensitive (ts) mutants of Rous sarcoma virus using crosses involving single and double mutants within the src gene. The order relative to the 3' end of a number of mutants has been determined, other mutants are being investigated at present. Using these well characterized ts mutants we plan to study recombination phenomenon by the marker rescue technique. Cell lines chronically infected by a defective strain of RSV are being superinfected with ts mutants and the frequency of wild type recombinants is measured. It is hoped that by relating recombination frequency to the site of the genetic lesion a model for genetic recombination for this system may be formulated. The second aspect of this research is to elucidate the mechanism of provirus formation using ts and non-conditional mutants of RSV. The problem we are investigating at present is whether at low multiplicity of infection one or more DNA proviruses are made.